Communication devices such as terminals are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communications system, sometimes also referred to as cellular radio systems or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within or connected to the cellular communications network.
Terminals may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area is being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or Base Transceiver Station (BTS), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also on the cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated at the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station or terminal. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station or terminal to the base station.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile cellular system for networks based on the GSM standard. UMTS is developed and maintained by the 3rd Generation Partnership Project (3GPP). UMTS uses wideband code division multiple access (WCDMA) radio access technology to offer greater spectral efficiency and bandwidth to mobile network operators.
In 3GPP Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
UMTS specifications allow for a UE to be configured into one of a number of so called Radio Resource Control (RRC) states. These RRC states comprise an Idle Mode state, denoted Idle state or Idle Mode, in which the UE has no active RRC connection with the network, and 4 connected mode states: URA_PCH, CELL_PCH, CELL_FACH and CELL_DCH. See 3GPP TS 25.304 v11.3.0, User Equipment (UE) procedures in idle mode and procedures for cell reselection in connected mode.
FIG. 1 depicts the UE RRC states. The states in the RRC connected mode, are Cell Dedicated Channel (CELL_DCH), Cell Forward Access Channel (CELL_FACH), Cell Paging Channel (CELL_PCH) and Universal Terrestrial Radio Access Network (UTRAN) Registration Area Paging Channel (URA_PCH).
CELL_DCH is different from all of the other states in that in CELL_DCH, mobility is actively managed by a Radio Network Controller (RNC) by means of handover and serving cell change procedures. In idle mode, CELL_PCH, URA_PCH and CELL_FACH states, the UE performs cell reselection based on measurements and general parameters that are configured by the network.
In order to perform cell reselection when necessary, a UE measures the received signal quality from its own cell, from neighbor cells and from cells on other Radio Access Technologies (RAT)s or carriers. In this context, “carriers” refers in general to other UMTS cells operating on different frequencies. The means by which the measurements are made depend on the type of UE and the configuration of the UE.
Measurements will typically be filtered using both Layer 1 (L1) and Layer 3 (L3) filtering. L1 filtering is a basic filtering of raw measurements performed in a physical layer of the UE. L3 filtering is a combining of several L1 filtered measurements. The L3 filtering coefficients and manner of the L3 filtering may be specified in the 3GPP specifications. The L3 filtering coefficients are configured at the UE by the network using RRC signaling. A measurement threshold is configured by the network by means of UE specific or cell specific signaling. The threshold may apply to received signal strength or received signal quality, such as e.g. signal to interference and noise ratio. In UMTS Frequency Division Duplex (FDD) the examples of signal strength and received signal quality measurements used by the UE in all RRC states, including idle mode, are Common Pilot Channel (CPICH) Received Signal Code Power (RSCP) and CPICH Energy per chip/Noise spectral density (Ec/No) respectively. An alternative to CPICH Ec/No is the received energy per chip divided by the power density in the band. See 3GPP TS 25.133 v11.5.0, “Requirements for support of radio resource management (FDD)”.
The UE procedures like identifying a new cell, acquiring or reading of System Information (SI) of a cell, such as e.g. cell information sent on Master Information Block (MIB) and System Information Blocks (SIBs, are also considered to be UE measurements. All these UMTS measurements are also interchangeably called mobility measurements or more generally Radio Resource Management (RRM) measurements. The cell to be identified or whose SI is read may belong to an intra-frequency carrier, an inter-frequency carrier or to an inter-RAT carrier e.g. an E-UTRA carrier. If a threshold is exceeded by a filtered measurement from a neighbor cell and/or a received signal strength or quality of a measured cell exceeds that of the cell on which the UE is camped, then the UE will change from camping on the old cell to camping on the new cell on which it has performed measurements.
Measurements may be categorized into 2 types. A first type comprises Intra frequency measurements, which are measurements that are performed on neighbor cells that use the same carrier as the cell on which the UE is currently camped. A second type comprises “Inter-frequency” and “Inter-RAT” measurements, which are performed on different carriers compared to the carrier of the cell on which the UE is currently camped. In inter-RAT measurements the carrier measured upon belongs to a RAT, which is different than that of the serving UMTS.
In idle mode, intra-frequency measurements may in principle be performed at any time by the UE. However the UE will be configured by means of cell or UE specific signaling with a so-called Discontinuous Reception (DRX) cycle in idle mode. The DRX cycle requires that during certain periods of time, the UE should have it's receiver switched ON in order to be able to receive signaling messages from the cell on which it is camped. During the intervening time, the UE may switch off it's receiver in order to reduce power consumption. Typically, measurements made for reselection evaluation on the same carrier are performed during the times at which the UE receiver has to be switched on according to the DRX cycle.
FIG. 2 illustrates allocation of time for gathering L1 and L3 filter samples for intra-frequency measurements. L1 samples are spot measurements made in the UE baseband and provided to the L3 filter. L3 samples are obtained by filtering the measurements reported by L1 after L1 filtering.
Inter-frequency measurements in idle mode may be performed at any time if the UE possesses at least 2 receiver chains, one of which can receive on the carrier of the camped cell and the other of which can be used for making inter-carrier or inter-RAT measurements. If the UE possesses only one receiver chain, then inter-frequency and inter-RAT measurements are performed by means of re-tuning the receiver to carriers on which measurements are to be made during periods of the DRX cycle in which the UE is not required to receive from the camped cell.
FIG. 3 shows Allocation of time for gathering L1 and L3 filter samples for inter-frequency measurements.
If the UE is configured in CELL_PCH or URA_PCH, the procedures for making measurements and performing cell reselection are similar to idle mode.
If the UE is configured in CELL_FACH, two possibilities exist for configuration of the UE to perform measurements for cell reselection. The first is that so-called “measurement occasions” are configured. If this is the case, then during such “measurement occasions”, the UE does not need to receive from it's camped cell and can therefore re-tune it's receiver to make inter-frequency or inter-RAT measurements. The second possibility is that DRX cycles are configured. In this case, the UE can make inter-frequency measurements at times at which it's receiver is not required to be tuned to the camped cell, in a similar manner to that of idle mode.
In order to guarantee good mobility performance, the 3GPP specifications place requirements on the maximum amount of time the UE should take to make all necessary measurements, including getting sufficient measurement samples for L1 and L3 filtering, in order to be able to make a decision that cell reselection is required.
In idle mode, the requirements are relatively loose and allow the UE a significant amount of freedom in scheduling it's inter- and intra-frequency measurements whilst still making it's decision within the required time.
In CELL_FACH mode, the requirements on the measurement time are more stringently specified due to the enhanced degree of connectivity. The CELL_FACH requirements are derived by setting first the total amount of receiver time that is required for getting sufficient measurement values for making a reselection decision. The requirement on the time during which measurements shall be performed then depends on the UE configuration. These requirements are pre-defined in 3GPP specifications and their compliance by the UE is ensured by means of conformance testing procedures. Examples of such pre-defined requirements, also denoted measurement requirements, performance requirements, RRM requirements etc., related to measurements used in CELL_FACH are cell identification delay, e.g. 10 seconds, physical layer measurement period or measurement time, e.g. 2 seconds, measurement reporting delay, measurement reporting time, measurement accuracy, number of identified cells required to be measured by the UE (e.g. 8 cells), applicable signal quality target for measuring a cell, e.g. CPICH Ec/No≧−17 dB; Synchronization CHannel (SCH) Ec/No≧−17; SCH Es/lot≧−4 dB; Cell-specific Reference Signal (CRS) Es/lot≧−4 dB etc. The CRS Es/lot is similar to CRS SINR. More specifically parameter Es and lot according to TS 36.133 are described as:
Ê s Received energy per RE (power normalized to the subcarrier spacing) during the useful part of the symbol, i.e. excluding the cyclic prefix, at the UE antenna connector.
lot The received power spectral density of the total noise and interference for a certain RE (power integrated over the RE and normalized to the subcarrier spacing) as measured at the UE antenna connector.
When measurement occasions are configured, the requirement on measurement time for inter-frequency and inter-RAT measurements is derived by allocating the time during which it is necessary for the receiver to be tuned to other carriers to measurement occasion periods such that enough measurement occasion periods are available for performing all of the measurements.
FIG. 4 shows derivation of the requirement for intra-frequency measurements in CELL_FACH.
When DRX is configured, the requirement for intra-frequency measurements is made by allocating the total time required for performing all measurements into the periods during which the receiver has to be ON and receiving from the camped cell.
The handling of inter-frequency and inter-RAT measurements when DRX in CELL_FACH is configured depends on the relative lengths of time of the receiver ON period and the period of time during which the receiver does not need to be tuned to the camped cell. The UE requires sufficient amount of measurement samples during the measurement time in order to meet the pre-defined measurement requirements, e.g. accuracy of the measurement, i.e. how accurate the measurement is. In a DRX cycle, the OFF time is the time during which the UE is not scheduled any data. However the UE may still use this time for doing measurements on radio signals, e.g. CPICH measurements. If the “Off” time is shorter than the “On” time, the time required for acquiring sufficient measurement samples for inter-frequency/RAT measurements over the measurement time is allocated to the “Off” times, i.e. OFF periods, when the receiver may be tuned to another carrier, and the requirement for reselection is derived by allowing a sufficient amount of time to collect all necessary samples on all carriers.
FIG. 5 depicts derivation of the requirement for inter-frequency measurements in CELL_FACH.
If the “Off” time is longer than the “On” time, then the time required for acquiring sufficient measurement samples is allocated to the “Off” times by assuming that the receiver is used for making inter-frequency/RAT measurements during “Off” times for a period of time as long as the “ON” time and allocating a sufficient number of “Off” time periods to allow the total required amount of measurement samples on all carriers to be collected or obtained between the “ON” time periods during the “Off” times. The requirement for reselection is derived as an amount of time required to cover the sufficient number of “Off” time periods.
FIG. 6 illustrates derivation of the requirement for inter-frequency measurements in CELL_FACH for a situation when the “Off” time is longer than the “On” time.
Typically, the UE is configured with a list of UMTS carriers and carriers of other RATs on which it should make measurements. In UMTS the other RATs or more commonly called inter-RATs can be any one or more of Evolved Universal Terrestrial Radio Access (E-UTRA) FDD, E-UTRA time division duplex (TDD), Global System for Mobile Communications (GSM)/GSM/EDGE Radio Access Network (GERAN)/Enhanced Data Rates for GSM Evolution (EDGE), Code Division Multiple Access (CDMA) 2000, High Rate Packet Data (HRPD) and even Wireless Local Area Network (WLAN). The examples of inter-RAT UE measurements used for cell reselection are Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ) in E-UTRA and GSM carrier Received Signal Strength Indication (RSSI) in GSM/GERAN/EDGE. An identification of an inter-RAT cell and acquisition of its system information, e.g. cell information sent on MIB and SIBs, are also kinds of inter-RAT measurements. All these inter-RAT measurements are also interchangeably called inter-RAT mobility measurements or more generally inter-RAT RRM measurements. However different carriers may be deployed by operators for different reasons. For example, a carrier might be provided with large cells for coverage reasons. Alternatively, a carrier might be provided for hotspots to add capacity.
For carriers necessary for coverage, it is only necessary for the UE to make measurements when the quality of its current cell is becoming poor. If the current cell is received with good quality, then it is not necessary for the UE to monitor carriers that are provided for coverage only and it can save UE battery life by not making measurements.
Hotspot provision, on the other hand is unpredictable from a UE point of view and hence it is necessary for a UE to continuously make measurements to evaluate if it should reselect to cells on a hotspot carrier.
Thus, the set of carriers and RATs on which the UE needs to make measurements differs depending on the quality of the current camped cell:                If the current cell is received with quality above a configurable threshold, then the UE makes measurements only on so-called “high priority” carriers and RATs, typically carriers provided for capacity. Measurements may only be made once per minute.        If the current cell is received with quality lower than a configurable threshold, then the UE makes measurements for evaluation of reselection on all carriers and RATs indicated to it. Measurements may be made continuously.        
For example, in all cases with CELL_FACH measurements, if inter-frequency or inter-RAT measurements are configured and made and then further inter-frequency or inter-RAT measurements are added to the configuration, then the measurement requirements for all of the existing and newly configured measurements are re-scaled to meet the existing time, which is already pre-defined in the standard, i.e. the time used by measurements without rescaling the measurements.
In idle mode, it is possible for a UE to autonomously reselect to a closed subscriber group (CSG) cell. The CSG cell may be accessed by selected set of the UEs also known as members of CSG. Typically a CSG cell is served by a customer premises node like home base station. Its access to the UE is therefore determined by the server provider or owner/subscriber of the CSG cell. The means by which the UE can detect the presence of a CSG cell, make measurements and reselect are left for UE implementation. However the UE is allowed to reselect a target CSG cell if this cell's CSG ID is included in the UE's CSG white list. Typically, the UE will need some sort of proximity detection. If the UE detects that it is in proximity to a CSG cell, then if the cell is on a different carrier as compared to those configured for reselection measurements the UE will need to make additional inter-frequency or inter-RAT measurements on the carrier of the CSG cell. It will need to schedule it's measurements such that it still meets the requirements for making reselection evaluation measurements. Since in idle mode the requirements on time durations for reselection measurements are relatively relaxed, it is straightforward for the UE to schedule the additional measurements on the CSG carrier.
In 3GPP Release 11, the specifications will allow for the UE to perform autonomous reselection measurements also in CELL_FACH.
In UMTS the network node, e.g. RNC may assign each UMTS carrier frequency and inter-RAT carrier frequency for a UE in a low activity RRC state, such as idle mode, CELL_PCH, URA_PCH and CELL_FACH, with a priority level ranging from 0 to 7. Thus a UMTS inter-frequency or inter-RAT carrier may be of low, equal or of higher priority with respect to the priority of the serving cell's carrier frequency. The inter-frequency/inter-RAT measurement rules, the cell reselection procedures and the corresponding measurement requirements in low activity RRC states thus also depend upon the priority levels assigned to the UMTS carrier and inter-RAT carrier frequencies.
As mentioned above, 3GPP has agreed to enable autonomous reselection to CSG in CELL_FACH. Further, as described above, the requirements on reselection evaluation in CELL_FACH may use all of the available measurement time in cases in which the length of the receiver (RX) OFF time in the DRX cycle is equal to or shorter than the RX ON time. If this is the case, then for some DRX configurations, it is not possible for the UE to schedule measurements on a CSG carrier for an autonomous reselection to CSG without compromising the existing measurement requirement, since the existing requirement requires the UE to use all of the available RX OFF time for performing such measurements. In other words for some DRX configurations the UE may not be able to meet the existing measurement requirements if it does not use the entire OFF time for doing measurements for reselection evaluation in CELL_FACH.
As mentioned before, the requirements on measurement time in CELL_FACH mode are more stringently specified than the requirements in Idle mode, due to the enhanced degree of connectivity. Furthermore, the OFF time periods of DRX mode, which may be used by a User equipment to perform measurements that require the User Equipment to tune its receiver to other frequencies, may be configured in different ways and may, depending on the length of the OFF time periods and the measurements to be performed, be a limited resource for fulfilling various requirements on measurement time.